1. Field of the Invention
The present invention relates generally to the field of frame synchronization in orthogonal frequency division multiplexing (OFDM) communication systems.
2. Description of the Related Art
Orthogonal frequency division multiplexing (OFDM) is a robust technique for efficiently transmitting data over a channel. This technique uses a plurality of sub-carrier frequencies (sub-carriers) within a channel bandwidth to transmit the data. These sub-carriers are arranged for optimal bandwidth efficiency compared to more conventional transmission approaches, such as frequency division multiplexing (FDM), which waste large portions of the channel bandwidth in order to separate and isolate the sub-carrier frequency spectra and thereby avoid inter-carrier interference (ICI). By contrast, although the frequency spectra of OFDM sub-carriers overlap significantly within the OFDM channel bandwidth, OFDM nonetheless allows resolution and recovery of the information that has been modulated onto each sub-carrier. Also, OFDM is much less susceptible to inter-symbol interference (ISI) from the use of a guard time between successive bursts.
Although OFDM exhibits several advantages, prior art implementations of OFDM also exhibit several difficulties and practical limitations. One difficulty with implementing OFDM transmission systems is that of achieving timing and frequency synchronization between the transmitter and the receiver. Another difficulty is achieving appropriate frame synchronization after some time and frequency synchronization has occurred. Locating frame boundaries in a slotted-ALOHA type control channel of an OFDM system is difficult because such systems are highly sensitive to frequency and timing errors.
In such a system, each base unit transmits a broadcast point-to-multipoint message during an allotted time such that all base units transmit a broadcast message within a period of time round-robin style. For frame synchronization, each receiver unit in the system receives the broadcast messages and must determine (1) the general location of the broadcast message within the frame structure; and (2) the broadcast message that originates from its particular serving base unit. These objectives are typically achieved in conventional systems by separate mechanisms. For example, item (1) usually requires a synchronization sequence as a frame marker. To detect the synchronization sequence, the receiver unit has to be synchronized in time to some degree. In OFDM systems, this requirement may be only a fraction of a digitized time sample, which thereby necessitates some complicated processing. Item (2) is typically performed by decoding some message broadcast by the base unit, which can only be performed after tight time and frame synchronization.
Accordingly, there is an existing need to provide alternatives to frame synchronization in OFDM communication systems. More particularly, there is an existing need to provide alternatives to frame synchronization that do not require the degree of time synchronization required in other approaches.
Methods and apparatus for use in obtaining frame synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) communication system are described. In one exemplary method the steps involve, for each time slot of a plurality of time slots within a duration of a message frame, where during some of the time slots each base unit simultaneously transmits a plurality of pilot tones in a predetermined pilot tone pattern and where during some other time slots each base unit does not simultaneously transmit the plurality of pilot tones in the predetermined pilot tone pattern: receiving OFDM signals within a time slot; determining a signal level in the OFDM signals at each frequency of a plurality of pilot tone frequencies; performing a correlation between the signal levels and the predetermined pilot tone pattern; and generating and storing a pattern detection value based on performing the correlation. The method includes the further steps of repeating, for a plurality of message frame durations, the above steps of repeated receiving, determining, performing, and generating to produce a plurality of averaged pattern detection values; performing a correlation between the plurality of averaged pattern detection values and a predetermined message frame pattern; and detecting a boundary of a message slot based on performing the correlation.
In another exemplary method, which is related to and combinable with the above-described method, the following steps are involved. For each message frame of a plurality of message frames during which each base unit takes a turn in transmitting a plurality of pilot tone signals: receiving OFDM signals within a message frame; determining a signal level of a plurality of pilot tones in the OFDM signals; and generating a rank of the plurality of base units in accordance with the signal levels. The method further includes repeating the following steps if necessary until a serving base unit is confirmed: selecting a candidate base unit from the rank; decoding a message within the message frame associated with the selected candidate base unit; comparing a base unit identifier in the message and a prestored base unit identifier; and confirming that the candidate base unit is the serving base unit based on a match between the base unit identifier and the prestored base unit identifier.